The present invention relates to a method for the production of molten metal by reduction of oxides of the metal. This includes production of molten iron, including pig iron and cast iron, as well metal alloys.
Reduction processes are intended to either produce steel directly from iron ore or make a product equivalent to blast furnace pig iron for use in conventional steel making processes, or produce low-carbon iron as a melting stock for producing steel by conventional processes. This process is generally intended to supplant blast furnaces as a source of molten iron production for steel making.
Blast furnaces typically constitute a vertical tower wherein a charge comprising iron ore, pellets or agglomerates, together with coke and limestone, are sequentially charged through the top of the furnace to form a continuous column of charge material. In the lower portion of the furnace, atmospheric air, which may be preheated, is introduced to the charge. When the charge materials come into contact with hot gases that are ascending from the hearth, the coke is preheated by these gases so that when it reaches the lower portion of the furnace and it comes into contact with the air introduced thereto, it will be caused to burn. At the resulting high temperatures existing at this location of the furnace, carbon dioxide is not stable and reacts immediately with carbon to form carbon monoxide. This reaction is not only the main source of heat for the smelting operation, but it also produces a reducing gas (CO) that ascends through the furnace where it preheats and reduces the iron oxide in the charge as it descends through the furnace.
The production capacity of a blast furnace is a function of the internal volume or area of the furnace design parameters for a given production capacity. Consequently, to increase capacity requires increasing the size of the blast furnace and accordingly adjusting the design parameters.
The present invention provides improvement over the above-described conventional blast furnace operation in the production of molten metal, particularly molten iron. Specifically, the method in accordance with the invention is used in association with a shaft furnace that may produce cast iron, pig iron or other metal alloys in a more cost effective manner than the use of conventional smelting operations, including blast furnaces.
The invention provides further advantage in allowing the conservation of fine materials from the top gases in the form of oxides of metals, such as zinc, cadmium and the like and permits the recovery and recycling of these metals and oxides.
The method for reducing metal oxides in accordance with the improvement of the present invention provides advantages over the conventional practices by the novel use of a shaft furnace for smelting. In this regard the method comprises reacting a charge of a metal oxide and a reductant to produce a primary molten metal of the metal oxide and gas containing carbon monoxide and an additional secondary metal and oxides, which are different than the primary molten metal and metal oxides. The gas is directed upwardly in the shaft furnace and away from this charge. The temperature of the gas is controlled at a location in the shaft furnace above the charge to be at a temperature that is higher than the condensing temperature of the secondary metal and oxides. This prevents the secondary metals and oxides in the gas from adhering to the interior wall of the shaft furnace. Thereafter as the gas passes upwardly and is removed from the furnace the secondary metal and oxides are removed from the gas. These may then be recycled in various ways, including using the same in the production of agglomerates for use in a charge to be refined.
The temperature of the gas may be controlled by varying the height of the charge within the shaft furnace. In addition the temperature of the gas may be controlled by varying the combustion rate of the gas by using a burner to heat the gas as the gas is directed upwardly within the shaft furnace.
The temperature of the gas may be additionally controlled by controlling the reaction rate of the charge.
The gas removed from the furnace consists essentially carbon dioxide and nitrogen.
The charge may include iron ore. In addition, the charge may include agglomerates that are self-reducing, self-fluxing, or both.
In conventional reduction practices, the reduction occurs by means of the CO generated from the partial combustion of the coke. The CO spreads into the charge with the reduction taking place according to the reaction Me+COMe+CO2. The CO2 gas generated in this reaction spreads in the opposite sense to the CO. This reaction requires a certain amount of time for the complete diffusion inside the charge. This requires furnaces with periods of residence times for the charge inside the furnace, which is typical of blast furnaces.
Self-reducing agglomerates, however, exhibit conditions that are significantly more favorable for reduction. The more intimate contact between the ore or the oxide and the carbon of the reductant (coal or coke) allows a shorter reaction time since there is no need for spreading the CO into the agglomerate. The reduction occurs according to the reactions below, and is preset within the agglomerate for this purpose:
2MeO+C2 Me CO2
CO2+C2 CO
MeO+COMe+CO2
The agglomerate itself thus establishes in practice a semi-closed system, wherein the atmosphere is a reducing atmosphere, during the entire period when carbon is available within the agglomerate. In other words, the self-reducing agglomerates, as implied by such designation, maintain within the same their own reducing atmosphere that is independent from the characteristics of the outer atmosphere, that is the atmosphere existing inside the shaft furnace provided by the ascending gasses.
It is therefore possible to convert into energy for the process the CO present in the atmosphere of the furnace provided by the partial burning of the fuel and the reducing reaction that takes place within the agglomerates and additionally allowing control of the temperature and the characteristic (oxidizing or reducing) of the top gasses.
In melting processes using shaft furnaces, the presence of coke or another fuel in solid form, charged through the top part of the furnace during the course of the operation, follows a descending path with the rest of the charge, reacting with the ascending CO2, in counter current relationship according to the reaction CO2+C2CO. This results in a greater consumption of carbon material, and thus prevents effective utilization thereof for the process of reducing/melting.
Due to the short residence time required for the self-reducing process in accordance with the invention, it is possible to operate the furnace according to the present invention with low charge heights. It is also possible to control the exhaust temperature of the top gasses and hence maintain in the form of vapor or fine particulate the oxides or metals that contaminate the residue used in the agglomerates. Hence, this material may be recovered at the gas scrubbing system. Because reduction of the content and the nonmetallic present in the residue, the fines recovered at the gas scrubbing system exhibit high concentrations of these oxides and metals, such as for example above 20%, rendering the subsequent recovery thereof economically feasible. Where the concentration of the oxides or metals is not found to have reached the desired level for economical recovery, it is possible to recycle these fines as many times as necessary by including the same in the production of the self-reducing agglomerates to increase the content thereof in the agglomerates and thus increase the concentration thereof in the recovered fines.